Updated 9:37 p.m. | Rep. Timothy Johnson (R-Ill.) will not seek re-election after all, according to a House Republican leadership aide.

His eleventh-hour retirement stunned many of his House Republican colleagues. Johnson made every indication he’d seek re-election in the mostly new territory, including securing the GOP nomination in the March 20 primary.

What’s more, Johnson’s decision puts his downstate Illinois seat in play. Democrats redrew Johnson’s district to be more competitive during redistricting last year.

The Illinois politics tip sheet, Capitol Fax, first reported Johnson’s retirement. According to the report, Johnson feels he can step aside now because he believes the seat is not competitive with emergency room doctor David Gill as the Democratic nominee.

Although officials are still counting absentee ballots in the close Democratic primary, Gill is on track to defeat Greene County State’s Attorney Matt Goetten. Democrats recruited Goetten to get into the House race, and local insiders viewed his defeat as a big blunder for the party.

Local Republican officials in the 13th district will now select a new nominee to replace Johnson, who was first elected to the House in 2000.

A senior Illinois GOP operative immediately pushed Rodney Davis, the former Illinois Republican Party executive director, as a potential successor to Johnson. The source also suggested Davis would have the support of his former boss, Rep. John Shimkus (R). Shimkus currently represents much of the redrawn 13th district but is seeking re-election in a nearby safe Republican district.

State Reps. Chapin Rose and Adam Brown as well as state Sen. Sam McCann are also considered potential candidates, according to Capitol Fax. Two of Johnson’s current and former top aides, former Chief of Staff Jerry Clarke and current Chief of Staff Mark Shelden, could also be contenders. Shelden did not immediately return a voice mail message and email seeking comment.

Updated 9:37 p.m.

Johnson prefers Clarke to be his successor, according to a top Illinois Republican source. Clarke currently serves as freshman Rep. Randy Hultgren’s (R-Ill.) chief of staff, and the source said he was one of only a few people Johnson called about his decision before the news broke.

House Republicans were stunned by Johnson’s decision. A loner in the delegation, Johnson did not tell top House GOP staff about his plans ahead of time. He scheduled a conference call with his staff Thursday, but the topic was unknown.

Replacing a nominee after the primary is not a unique situation in Illinois. In 2006, then-Rep. Lane Evans announced his retirement in late March after winning the Democratic nomination. Local party officials tapped Evans' top district aide, Phil Hare, to be their nominee and he went on to win in November.

Johnson is the 24th House Member to retire and not seek higher office this cycle. To see who else is leaving or trying to move up, check out our Casualty List.